1. Field of the Invention
A system for maintaining the specific cake resistance substantially constant by controlling the quantity of filter aid fed to a sediment filter.
2. Introduction
The separation of solids from liquids is one of the most widely used operations in industrial processes. The material recovered after separation either may be the liquid or the solids, depending on the desired end product. Filtration is one of the most commonly used methods to achieve this separation. One type of filtration frequently referred to as cake filtration, uses filter aid, being a two-stage operation. In the first, a thin layer of filter aid, e.g. a fine, liquid-insoluble powder such as diatomaceous earth, commonly known as a "precoat", is built up on a filter septum by adding to a clear liquid circulating through the filter system an adequate quantity of filter aid (usually 0.1 to 0.2 lbs/ft.sup.2 of septum). After precoating, small amounts of filter aid (body feed) are regularly added to the liquid to be filtered. As filtration progresses, the suspended solids from the unfiltered liquid are retained in the filter by entrapment in the porous structure of the cake formed by the body feed particles. In the absence of body feed the imprevious solids carried by the liquid would rapidly slime over the filtration surface and flow rates soon would drop to unacceptably low levels. The regular addition of body feed, by providing a continuously renewed filtration surface, results in a dramatic increase of filtration efficiency. The effect of the rate of body feed, for a certain level of sediment solids in the unfiltered liquid, is as follows: Too slow a rate of addition reduces the total throughput since the body feed particles, being completely surrounded by the impervious solids fail to increase the permeability of the filter cake formed. The thickness of the filter cake increases slightly, without improvement in its porosity. After a certain point, however, the solid particles will not fill all the voids and pores in the filter cake and the porosity of the cake will improve with consequent increase in total attainable throughput. From this point on, as the fraction of pores available for liquid flow increases, the throughput will continue to increase. First steadily for awhile, but then tapering off. It reaches a peak after which it will start to decrease slowly. This happens when the improvement in permeability does not compensate for the increase in resistance produced by the additional body feed. It would seem, at first glance, that the rate of body feed corresponding to the mentioned peak should be the ideal for conducting in particular filtration. This is usually not so, however, since it corresponds to a condition of clear overfeeding (at least for filtration of liquids with a viscosity similar to water, like wine, beer, etc. and for the degree of porosity of the filter aid grades most commonly used).
The concept of the optimum rate of body feed is better understood by close analysis of the two extreme filtration conditions: underfeeding and overfeeding. The first, resulting from too slow a rate of addition of filter aid, leads to a very rapid increase of the cake resistance. The filtration cycle will be shortened due to the rapid flow rate drop. Increasing the pressure is seldom possible because the pressure in the filter cannot be raised above a certain safety limit. The second condition, resulting from too rapid a rate of addition of filter aid, will also reduce the maximum attainable throughput per cycle due to a rapid increase in the cake thickness. This necessitates premature interruption of the filtration when the filter space limitation for cake buildup is reached resulting in the danger of cake bridging, and possibility of equipment damage.
Since, in an extreme situation of the first condition, a rapid "clogging" of the precoat surface may result, the tendency usually is to overfeed. The filtration is usually started with an over-generous rate of body feed followed by tentative and empirical adjustments as the filtration progresses. The rate of pressure increase and/or flow rate drop across the filter are the measurements that help the operator in deciding whether adjustments are needed. Observation of the unfiltered liquid turbidity is another clue as to the concentration of sediment solids and may forewarn the operator as to the need for body feed changes.
The known systems that attempt to automate part of the manual operations associated with the rate of body feed consist mostly in the utilization of solids/liquid proportioning devices whose object is to keep constant the chosen rate of filter aid feed regardless of flow rate variation. Since these systems cannot otherwise react to changes of solids concentration in the unfiltered liquid, further controls are frequently used consisting of the utilization of turbidity instruments in the in-feed line. A signal from the turbidity measurement is used to automatically adjust the proportioning factor in response to solids concentration changes. To compensate for color interference, a second turbidity measurement in the filtered liquid line is usually performed.
The difficulties associated with these systems arise from two main problems. The first one is a consequence of the poor relationship between turbidity and solids concentration. The second and most troublesome results from the often poor correlation between solids concentration and filter aid body feed requirements. This is so because the physical properties of the solids are usually of greater importance than its mere concentration.
3. Purposes of the Invention
It is an object of the present invention to provide a system that has an efficient automatic control of filter aid feed rate, thus circumventing the above mentioned prior art drawbacks.
Another object of the invention is to provide a filtration cake with physical characteristics that can be measured during the process and that directly relate to filtration efficiency, namely a "practical specific cake resistance".
Still another object of the invention is to provide methods to determine the practical specific cake resistance value that best approaches the most efficient filtration conditions.
It is a further object of the invention to provide a new system that automatically will maintain the aforesaid preselected most desirable specific cake resistance.
4. Description of the Prior Art
One of the recent inventions that attempts to solve some of the previously described difficulties is disclosed in U.S. Pat. No. 4,118,778 (STRUB), granted Oct. 3, 1978. To derive the control signal for filter aid feed adjustment, the STRUB system utilizes measurements of variables more closely related to filtration conditions, namely flow rate and differential pressure across the cake. Inasmuch as these variables are a direct effect not only of the solids concentration, but also of the physical properties of the solids, this is an improvement over the described prior art. Unfortunately, the system fails to achieve the full advantage that should be expected from that improvement due to the manner in which the control signal is derived from the measured variables. Since in some aspects the STRUB system may superficially resemble the instant invention, it will now be discussed in more detail.
The object of the STRUB invention is to obtain a "substantially even loosening up" of the sediment over the entire cake. The inventor proposes to achieve this by making the quantity of filter aid delivered per unit of time dependent on a control signal J.sub.3. The value J.sub.3 is generated from the variable q which is the flow rate through the cake and p which is the differential pressure across the cake, in two different proposed ways, namely: EQU J.sub.3 .about.P/Q with Q=.intg.qdt (1)
In this first alternative, it is evident that the rate of filter aid feed (.about.J.sub.3) for the incoming liquid is made dependent on information P/Q only (and entirely) derived from filtration conditions resulting from the liquid already filtered. This is past history and, therefore, useless to adequately handle solids load variations in the liquid input to the filter cake. Therefore, in order for this system to assure an "even degree of loosening", it would be necessary that the concentration of sediment in the in-feed liquid remain constant, an ideal condition not usually present in actual practice. This is a tremendous limitation and excludes one of the principal objectives of applicant's invention, viz, the handling of sudden changes in the quantity of incoming sediment.
Another limitation of the STRUB invention is that, in the above expressions, q must necessarily represent not only the flow rate across the cake, but also the flow rate of the infeed liquid (q must not only carry the information relating to the differential pressure developed across the cake but also the one pertaining to the total volume of liquid carrying the sediment solids). In applicant's system, these two flows do not have to be necessarily the same and in some conditions, they are required to be different. (see the function of the automatic circulation valve in applicant's system as described hereinafter). ##EQU1##
Intuitively, it seems correct to expect an "even degree of loosening" if the rate of filter aid feed per unit of time is made proportional to the rate of cake resistance increase, per unit of time. However, what seems to be feasible runs into unsurmountable problems in practice; these result from the difficulty of performing a prompt and adequate determination of the value ##EQU2## from the inherently excessive noisy measurement of the cake resistance. STRUB acknowledges these difficulties (STRUB Patent, Col. 12, lines 16-43) and tries to overcome them by eliminating the random noise from the resistance value by integration during successive time intervals varying from one minute at the beginning of filtration to two minutes toward the end. The time derivative value is then calculated by dividing the two last successive integrated values by the average time interval. This is done in digital form and the result is then converted into an analog signal.
This second method suffers from a grave shortcoming. The need to use integration times of the order of one to two minutes results in a slow response of the control system and consequent inadequate response to sudden changes in solids load. On the other hand, for liquids of low solids concentration, integration times of even two minutes will be too short. This results in the integration being meaningless due to the very slow rate of resistance increase.
This explains why STRUB admits that extensive experimental investigations failed to reveal that this second method has any real advantage over that first described (STRUB Patent, Col. 8, lines 50-60).
From the above considerations, it will be seen that applicant's invention presents, in relation to STRUB's, the following clear and innovative improvements:
1. Applicant's control system is designed so as to produce a filter aid cake with a constant, preselected "effective" specific resistance. This relates to a filter aid cake physical property that is defined and claimed in clear and practical terms in order to make its measurement, during filtration, easy and accurate. This contrasts with STRUB's vaguely stated aim of obtaining an undefined "substantial even loosening up of sediment" through the entire cake (STRUB Patent Col. 9, lines 44-46).
2. Applicant describes a method of determining what value to select for the specific resistance so as to achieve the best balanced feeding (maximum volume output per cycle). It is obtained by calculation and is expressed by a numerical value. In the STRUB system, the "degree of loosening up" is dependent on the proportionality factor between the control variable (output of calculator unit 6, STRUB FIG. 1) and the quantity of filter aid feed per unit of time. What that factor should be, in numerical terms, is never stated. As a matter of fact, STRUB's claims 25, 26 and 27 are admissions that his system is incapable of assuring an "even loosening up" since they call for a "rigidly predetermined quantity of filter aid per unit of time" to be fed at beginning of filtration, and suggested that the value should correspond to the middle range between the minimum and the maximum rates of filter aid dosage (STRUB Patent, Col. 39, lines 35-59). Also, the same admission is evident in the suggested method of how and when to adjust the proportionality factor, during filtration (STRUB Patent, Col. 36, lines 2-8).
3. In applicant's system, the specific cake resistance is determined and controlled in a direct way, without any need for integration time intervals. This eliminates the response delay intrinsic in STRUB's system which is the main reason for the shortcomings of his system. In fact, the perception that the ratio between the rate of increase of resistance per unit of time to the rate of filter aid per unit of time determines the degree of loosening up is correct, but only if both time bases coincide.
4. The handling of hydraulic distrubances, other than random noise, through an "equalizer" control functions is a feature of great convenience in the presently disclosed filtration control system not provided by STRUB's system.
5. An automatic circulation valve is another feature dramatically extending the range of sediment solids concentrations possible to be handled by applicant's system. This is also not available in STRUB's system.